Abstract
The aim of this study is to investigate the capability of a 6 DoF parallel robot to perform various rehabilitation exercises. The foot trajectories of twenty healthy participants have been measured by a Vicon system during the performing of four different exercises. Based on the
kinematics and dynamics of a parallel robot, a MATLAB program was developed in order to calculate the length of the actuators, the actuators’ forces, workspace and singularity locus of the robot during the performing of the exercises. The calculated length of the actuators and
the actuators’ forces were used by motion analysis in SolidWorks in order to simulate different foot trajectories by the CAD model of the robot. A physical parallel robot prototype was built in order to simulate and execute the foot trajectories of the participants. Kinect camera was used to track the motion of the leg`s model placed on the robot. The results demonstrate the robot’s capability to perform a full range of various rehabilitation exercises.1. Introduction
Stroke is a leading cause of death [1] and it is also a leading cause of chronic disability with
100,000 patients having their first stroke annually in the UK. Although the rate of mortality
due to stroke has been falling, the prevalence of stroke is expected to increase in the future
due to the aging population [2]. The improvement of motor recovery and motor plasticity of a
patient along specific patterns is the aim of rehabilitation exercises [3-6]. The use of robotic
technology in rehabilitation can accelerate the treatment and recovery of the disabled and it
actuates the rehabilitation clinics to change their path from labour-intensive operations to
technology–assisted operations [7]. Emerging robotic technology in a traditional
rehabilitation therapy session would provide high quality treatment at a lower cost and effort.
The level of motor recovery of patients can be quantified by defining different rehabilitation
exercises for the robot [8].
Different lower limb rehabilitation-assisting robots have been developed to revive the
functional mobility of damaged limbs, ranging from complex computerized stations to simple
structures. These systems have been categorized in five different groups:

(i) treadmill gait trainers,

(ii) foot-plate-based gait trainers,

(iii) over ground gait trainers,

(iv) stationary gait trainers,

(v) ankle rehabilitation systems.

In some studies resistive force has been provided
during exercise and haptic simulation has been interacted with VR simulation [9,10]. Using a
6DoF parallel robot is an applicable method for lower limb rehabilitation due to its simple
configuration and high flexibility in performing a different range of motions [10]. Rutgers is
one of the pioneering ankle rehabilitation devices and its developments have been cited in
eight different studies [11-17]. In another study, a high performance 2DoF over-actuated
parallel mechanism has been designed and built for ankle rehabilitation based on custom
designed backdrivable actuators and an impedance control system [19-21]. In another study,
the prototype of a 3-RSS/S parallel mechanism has been produced for ankle rehabilitation
application [21, 22], but no clinical trials have been found for this system.
To develop the robotic devices, first the motor learning and motor adaption of healthy people
should be investigated; and then the neurologically injured patients can be rehabilitated with
respect to the obtained results [23, 24]. Therapeutic exercises can vary from range of motion,
active assistive isotonic, isometric, isokinetic and manual exercises [25-27]. The path
planning of a hybrid parallel robot for ankle rehabilitation was investigated based on inverse
kinematics during normal walking and stepping [28-30]. The 6DoF parallel robot is stiffer
than a tripod as it employs an extra three actuators [31]; hence it requires actuators with lower
load capacity.
There has been multiple clinically-evaluated systems using Kinect, but the majority are being
used for upper limb rehabilitation, as seen in [32-34]. Kinect would only be a viable
alternative to current red, green, blue, depth (RGB-D) cameras if it provided accuracy that
was comparable. In [35] they investigated the accuracy of Kinect by comparing it to a Vicon
camera, which is a very precise, but expensive and bulky, marker-based motion capture
system. In [35] Kinect is distanced between 1.0m and 3.0m from the targets and the RMS
errors between the Kinect and Vicon system are all below 10mm. In [36] it is reported that
although Kinect had varied success depending on the activity being done, there was still a
good overall relationship between the results of the Kinect and the Vicon system for most
movements. In [37] the Kinect is once again compared to the Vicon system, but this time it is
tracking elderly persons’ foot movements; it is reported that it provides acceptable accuracy
in measuring variation in stride velocity [38].
In this paper the healthy participants’ data has been used to evaluate the capability of a 6 DoF
parallel robot during four various rehabilitation exercises. The performance of the robot
based on real patient data during normal walking will be investigated in a separate research
study. The aim of this study is to investigate and evaluate different characteristics of the
parallel robot during the performance of different exercises for the rehabilitation of a lower
limb. The CAD model and a physical prototype of a 6DoF robot have been designed and built
to simulate the predefined foot trajectories of healthy subjects. All of the required force will
be supplied by the actuators during the performing of the exercises. A Kinect camera was
used as a depth motion sensor to detect the position of the robot`s end effector during its movements.